As a result of their high surface area, large pore volume and/or pore size selectivity, porous polymer structures have found wide application in many technological fields. For example, porous polymers may be used as separation or filtration materials, as absorbent materials and as scaffolds for catalysis, immobilisation of pharmaceutical compounds or biological molecules and tissue engineering.
The wide applicability of porous polymer structures is also very much reliant upon the different physical and chemical properties of the polymer materials from which they can be derived. For example, to enable porous polymer structures to function effectively in liquid separation and filtration applications it will be important that they are formed from polymer materials that are not chemically degraded or dissolved by the contacting liquid(s). The polymer material should also maintain adequate mechanical properties upon being exposed to the contacting liquid(s). In the case of tissue engineering applications, it will generally be important that polymer materials are biocompatible, and it may also be desirable that the polymer materials maintain good mechanical properties in vivo for a considerable period of time but subsequently biodegrade.
Despite there being a diverse array of polymers with different physical and chemical properties that may be used to form porous polymer structures, there are invariably applications that would best be served using a porous polymer structure having properties characteristic of two or more different polymer materials. For example, it may be desirable that a porous polymer structure has the rigidity of polystyrene and the hydrophilicity of polyacrylic acid. An idealistic approach to solving this problem might be to prepare porous polymer structures from a blend of two or more different polymers, for example a blend of polystyrene and polyacrylic acid. However, those skilled in the art will appreciate that most polymers do not often form a compatible polymer blend. In other words, blending different polymer materials generally results in the formation of an immiscible polymer blend.
Immiscible polymer blends are typically characterised by a phase separated morphology. The nature of the phase separated morphology is primarily dictated by the relative proportions of the polymers present. For example, a phase separated polymer blend of approximately equal proportions of polymers A and B will generally provide a bi-continuous morphology. Where the proportion of polymer A in the immiscible polymer blend is higher than polymer B, the polymer blend will generally have a discontinuous morphology with polymer A being the continuous phase and polymer B being the discontinuous phase. Conversely, where the proportion of polymer B in the immiscible polymer blend is greater than polymer A, the polymer blend will generally have a discontinuous morphology with polymer B being the continuous phase and polymer A being the discontinuous phase.
Despite comprising polymers that may individually have desirable properties, immiscible polymer blends often exhibit inferior properties relative to the individual polymer components alone. Such inferior properties are believed to stem primarily from minimisation of the interfacial surface area between the immiscible polymers during phase separation which affords relatively coarse phase separated domains. The coarse phase separated domains of the blend in turn result in interphase interfaces that are compositionally sharp and mechanically weak. The inferior properties of such immiscible polymer blends are often exacerbated when they are formed into high surface area products such as porous polymer structures.
Numerous techniques for improving the properties of immiscible polymer blends have been developed. Perhaps the most effective and widely studied of these techniques has been the use of compatibilisers to modify the phase separate morphology of the polymer blends. Compatibilisers are generally polymeric materials that have a degree of miscibility in each of the phase separated domains and can therefore function as a bridge between them. This bridging function reduces the interfacial energy between the domains and enables them to be more finely dispersed or intermixed. This in turn improves the properties of the resulting blend.
Although the use of compatibilisers can promote better intermixing between immiscible polymers to afford blends that exhibit improved properties, each compatibiliser is typically blend specific and not suitable for use with other polymer blends. Furthermore, the process of developing an effective compatibiliser for a particular polymer blend can be quite difficult.
It would therefore be desirable to develop a method for preparing porous polymer structures from immiscible polymer blends that was not reliant upon the need to use compatibilisers, and yet could promote intimate mixing between the immiscible polymers to impart improved properties to the porous structure.